Security printing

ABSTRACT

A method of providing covert security features for documents such as vouchers, packaged goods and banknotes in which the document is provided with a dopant. The dopant consisting of a material which can be identified by examination of its response to visible wavelength photon radiation and which can be applied directly on or into the document or can be fused into glass matrices before application.

This application is a 371 of PCT/GB99/03692 filed on Nov. 8, 1999.

The invention relates to materials and techniques relating to securityprinting.

The present invention in its broadest sense is concerned with theprovision of security in relation to documents, vouchers, packaged goodsand tokens of value. Examples of these are banknotes, cheques anddrafts, bond and stock certificates, and credit and bank cards. All ofthese are referred to hereinafter for simplicity as “documents”.

Documents of this nature have the requirement to be as secure aspossible against forgery and falsification and for this purpose it isdesirable that they exhibit both covert and overt security features. Theexpression “covert security feature” is used to denote some securityfeature which is not visually apparent to the normal user, whereas“overt security feature” is used to denote a feature which can bereadily seen and recognised by members of the public without the use ofspecialised equipment or confidential information. Traditional forms ofovert security features include water marks, metal security threads, andthe use of specialised forms of paper and printing.

Known methods of covert security include NIR and IR absorber inks,magnetic threads, complex optical and electrically conductive indicia,anti-Stokes, visible-wavelength-emitting phosphors etc.

With rapid advances in reprographic technology such as relatively cheapand high quality colour photocopiers and easily available digital imagemanipulation, the traditional forms of security have become increasinglyeasy to circumvent. This is because the absorption and emission in thevisible, NIR and IR ranges of all the currently used and proposedsecurity dopants are readily available in the public domain since thecurrent materials were developed for the laser and lamp industries. Thisis particularly true for all the rare earth containing absorbers andemitters, where many thousands of public domain references of absorptionand emission spectra are listed from the 1950's onwards. There isaccordingly, a requirement for improved forms of both covert and overtsecurity features, preferably ones which can be used with existingprinting technology at modest cost.

According to one aspect of the present invention, there is provided amethod of providing a document with a covert security feature, in whichthe document is printing using an ink containing a dopant, the dopantbeing of a material which can be identified by examination of itsresponse to visible wavelength photo radiation.

This and other aspects and features of the present invention are definedin the appended claims.

The present invention will new be described by way of example withreference to the accompanying drawings of which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a blue ink reflectance spectrum from a paper print;

FIG. 2 shows green ink reflectance spectrum from a paper print;

FIG. 3 shows red ink reflectance spectrum from a paper print;

FIG. 4 shows a reflectance spectrum from the Praesodymium Oxide dopantin accordance with the present invention;

FIG. 5 shows a reflectance spectrum from the Neodymium Oxide dopant inaccordance with the present invention;

FIG. 6 shows a reflectance spectrum from the Holmium Oxide dopant inaccordance with the present invention;

FIG. 7 shows a reflectance spectrum from the Thulium Oxide dopant inaccordance with the present invention;

FIG. 8 shows a reflectance spectrum of raw Europium Oxide powder as usedin the present invention;

FIG. 9 shows a reflectance spectrum of the same European Oxide containedin glass;

FIG. 10 shows a reflectance spectrum of raw Erbium Oxide powder as usedin the present invention;

FIG. 11 shows a reflectance spectrum of the same Erbium Oxide containedin glass;

The present invention provides a range of inorganic dopants designedwith absorption spectra sufficiently different in form and structurefrom the absorption spectra of printing inks so that the dopants can beeasily identified. They thus become very covert because they exhibit noUV, visible or IR stimulated output to be observed by a counterfeiter.

The preferred elements for our dopants can be fused with other elementsin order to hide the presence of the dopant element, or to alter itsabsorption spectrum; or the oxide or salt of preferred element itselfcan be directly mixed into, for example, a printing ink or a batchcomposition for plastics production etc. When the dopant is mixed withother elemental compounds and where one of its admixture compoundscontains a substantial proportion by weight of a particular range ofatomic number (z) elements, varying the proportion of this compound inthe final mix can vary the absorption spectrum of the final inorganicmixture, thus essentially creating further dopants.

The present invention depends on the incorporation of a synthesisedinorganic dopant into or onto the document at any stage of itsmanufacture, including the printing stage. These dopands are designed tohave very complex visible wavelength absorption spectra, measured ineither reflective or transmissive mode. The spectra they exhibit are notfound in printing inks or common marbling substrates. This results inhigh signal-to-noise ratio detection, and hence the ability to identifythe dopant in 10 msec or less using low output (c. 4W) bulbs asilluminants.

The dopant incorporation with its unique spectrographic pattern givesindependence from document soiling, wear and tear etc, because it allowsexcellent signal-to-noise ratio. Pattern recognition software toidentify, within 1 msec, the complex signature of our synthesiseddopants is readily available from suppliers in the public domain, havingbeen used in optical and nuclear spectrometry for 30 years. Dopants inaccordance with the present invention can be incorporated singly, mixed,or in separate areas to produce a “bar code”, or to simply confuse aforger. The dopants, depending on composition, are either colourless ortransparent, or coloured, at the choice of the user. Dopants made inaccordance with the present invention provide high optical absorptionyet give optical transparency because their absorption features arecreated at wavelengths to which the human eye is insensitive.

For visible wavelength interpretation the preferred method is toilluminate an area of at least 5 mm² by a ring of at least 6–8 200 μoptical fibres in a concentric ring, and channel reflected light throughan inner 200 μ optical fibre to the wavelength detector. It has beenfound that this number of optical fibres gives sufficient signal forinterpretation of the spectra, however the present invention is notlimited to this method of detection of the spectrum or the number orarrangement of optical fibres used in this detection method. Thiseliminates the optical losses due to lenses in much prior art, which inturn leads to the processing speed of our system. CCD based wavelengthdetectors, followed by A-D conversion for processing are standardtechnologies in public domain electronics. Our dopants are engineered togive no visible signal, such as fluorescence, upon illumination by UV,visible, or IR radiation and are hence not easily replicated as hashappened with fluorescent inks, and other emitting technologies.

The advantages of the present invention will be readily apparent whenthe spectra obtained from these dopants is compared with those obtainedfrom standard printing inks, or colourisers in plastics etc. Thestandard inks and the like give relatively unsophisticated reflectancespectra—see for example FIGS. 1, 2, 3. These show the visiblereflectance spectrum of a Pantone standard blue, green and red ink froma paper print. FIGS. 4, 5, 6, 7 show the visible reflectance spectrafrom the four dopants, Praesodymium Oxide, the Neodymium Oxide, theHolmium Oxide and Thulium Oxide, incorporated in a clear litho varnishand printed on the same paper as that used to obtain the spectra shownin FIGS. 1, 2 and 3.

The prints obtained using dopants in accordance with the presentinvention are completely colourless to the eye. FIG. 4 for example,shows many easily identifiable peaks, troughs and turning points in itsspectrum with a shape easily distinguished from any ink or colouringdopants. It is these unique features which give the excellentsignal-to-noise ratio, giving the rapid identification ability of oursystem, with excellent identification rates, and very low falseacceptances, together with high rejection for forged copies.

The features, and/or slopes, of the reflectance spectra can be shiftedto create other dopants by incorporating the dopants into inorganiccompounds of the type described later.

The use of visible wavelength spectrometry, as opposed to IR or NIRwavelengths, makes possible many more commercial applications. This isfirstly because of the reduced cost of components for the visible, andsecondly because the cheapest excitation source is a common (4W) torchbulb which emits plenty of visible light but very little IR. Hence IRand NIR techniques require more powerful and costly excitation sources.Also by moving to the visible we make it easy to construct simplehand-held portable instrumentation which again increases possiblecommercial applications.

Visible wavelength spectroscopy as revealed in the prior art withapplication to security uses lenses or mirrors and lamps to provide theillumination source.

Many suppliers, such as Oriel Corp. USA, now make commercially availablereflectance probes which are about 6 mm diameter overall and contain aring of illuminating fibres (200 μ diameter 6–8 in number) surrounding acentre core of detecting fibres. Use of these probes gives much improvedsignal-to-noise ratio at the CCD array, or Si photodiode array, or otherdetector. Using other off-the-shelf components the output of the arrayspectrometer can be coupled to D-A converters and operated from alaptop, hand-held palmtop, or desktop PC computers. This can easily beinterfaced to standard computer software on production lines forauthentication at high speed—10 m/sec.

The dopants we have identified as working well can be added to standardoffset litho printing inks in a manner known to those skilled in theart. It is added in quantities up to about 30% by volume withoutaffecting the printing process, providing the dopants have beenmicronised into fine powders of the order of 1–4 μm diameter. If thisstep is omitted poor uniformity printing results. Our dopants need addno colour to the ink, so give a colourless invisible printed strip ontothe object to be protected. Alternatively a colouring dopant can beselected to blend in with an existing coloured scheme.

A major advantage of the dopants made in accordance with the presentinvention is that they are cheap and simple, not requiring the presenceof complex expensive chemicals.

The dopants can be applied to artefacts by any standard depositiontechnique—air spray, lacquering, printing, stamping.

The dopants could also be directly incorporated into paper or plastic(for example) at time of manufacture of said paper or plastic. For ourtechniques to work it is not necessary that the dopants are added as asuperior layer or film, although in many cases this will be the simplestand cheapest method. The fact that our dopant/excitation/detectortechnology does not require surface deposition can offer moresecurity/covertness to the process. It arises because the excitationmethods we are employing have ranges of many tans of microns in commonmaterials such as paper and plastics. Since dopants in accordance withthe present invention need not be on the surface of the document theforger is denied the opportunity to scrape off samples from repeatedsmall surface areas and analyse them to look for “surprising” changes incomposition from area to area. Such changes give the forger a clue thatcovert technology is being used in that area.

The multiple peaks, troughs, and turning points resulting give rapid,positive, unambiguous identification of dopant presence (and henceobject authenticity) and allow multiple dopants to be used as a furthermethod of disguise, if required.

The preparation of the inorganic powders for doping to permitidentification by visible light is not limited to the use of chemicalcompounds which could be formed by precipitation from a solution becausesuch compounds are limited in numbers. It has been found that the mostuseful compounds (those with the most distinctive absorption spectra inthe visible) could be formed by fusion melting. Silicates, phosphates,borates have been found to be the most useful starting points forfusion, because they give transparent glass matrices.

In forming the required solids for powdering, the chemical batchcomposition is not, for example, limited to that required to produce,say, a glass. This is because long range atomic order to is not requiredin the solid, since homogeneity is assured by micronising thecomposition. Indeed in general terms we have found that the bestcompositions are obtained where phase separation of the melt temperatureis imminent. This point is determined experimentally for eachcomposition. Nor need the chemistry be limited to stoichometric ratiossuch as to arrive at crystalline compounds, e.g. as used to produce thecommonplace inorganic fluorescence powders added to printing inks.

In many compositions, the structure and magnitude of the absorptionpeaks can be controlled over a wide range by control of the gasatmosphere during the melt phase. This is established by trial and errorfor each composition by test melting each composition in air, in areducing atmosphere, and in an oxidising atmosphere to determine theoptimum methodology and conditions for the absorption profile required.

In many compositions, the structure and magnitude of absorption peakscan be controlled by including a substantial quantity (>20% by weight)of a high atomic number Z element in the batch composition (lanthanum,bismuth, and strontium work well, as examples). Then varying the contentof this high Z element only gives changes in position and magnitude ofthe absorption peaks, from composition to composition. Differentabsorption peak wavelengths and magnitudes from that exhibited by theraw dopant before being incorporated in a glass.

The effect of incorporating the dopant in a glass on its spectrum can beseen in FIGS. 8, 9, 10 and 11.

FIG. 8 shows a plot of the percent transmission against wavelength (nm)for a raw Europium Oxide dopant powder. FIG. 9 shows a plot of thepercent transmission against wavelength (nm) for a Europium Oxide dopantpowder incorporated in a glass and ground into a fine powder. Thesubstances contained in the glass are as given in Table 1 below and theglass plus dopant is made in accordance with the method given belowTable 1 on page 14.

Simply from visual inspection it can be seen that the two spectra arevery different.

The feature of the spectrum of Europium Oxide shown at reference numeral81 for the raw oxide powder that occurs at a wavelength of 533 nm hasbeen shifted to 531 nm. A similar shift can be seen for the lowerwavelength peaks 83 and 93. In both cases, the shift in wavelength was 2nm. The most significant difference between the spectra of FIG. 8 andFIG. 9 is the presence of the line in the spectrum of the Europium Oxidecontained in glass at 393 nm. There is no similar line in the raw powderspectrum.

FIG. 10 shows a plot of the present transmission against wavelength (nm)for a raw Erbium Oxide dopant powder. FIG. 11 shows a plot of thepercent transmission against wavelength (nm) for an Erbium Oxide dopantpowder incorporated in a ground fine powder glass. As with the sampleused to obtain the spectrum if FIG. 9, the substances contained in theglass are as given in Table 1 below and the glass plus dopant is made inaccordance with the method given below Table 1 on page 14.

FIG. 10 shows, at reference numeral 101, the existence of multiple peakstructure occurring from a minimum point at 654 nm to approximately 700nm. It can be seen that these features are absent from the spectrum ofFIG. 11 as indicated at reference numeral 111.

FIG. 10 also has multiple peak structure occurring from a minimum valueat 521 nm up to approximately 600 nm. These features are absent from thespectrum of FIG. 11 as can be seen at reference numeral 113.

We have shown our dopant technology to work in a wide variety ofcompounds, including, but not limited to, silicates, borosilicates,borates and germanates.

The following are a number of examples of the composition and method ofmanufacture of a doped glass in accordance with the present invention.

EXAMPLE 1

A glass batch of a typical suitable composition is as follows.

TABLE 1 Compound Wt % SiO₂ 35% B₂O₃ 40.0 Na₂O 8.5 K₂O 8.5 Al₂O₃ 1.0 MgO4.0To this batch was added 0.1 to 25 wt % of Eu₂O₃. All powder sizes can beused but approximately 250 mesh is preferable. A wide range of cruciblescan be used, a Platinum crucible was used in this case. The final batchis mixed and homogenised that it is added to the crucible heated to 845°C. The temperature is then increased at a rate of approximately 5°C./min to 1200° C. the final melt temperature. It has been found thatgood quality melts are produced by holding the melt at the finaltemperature for between 2 and 2.5 hours before powdering the glass. Forabsorber products not visible to the naked eye, the natural emissions ofEu₂O₃ may be quenched by the use of high concentrations of Eu₂O₃ or bythe inclusion of small <1% quantities of nickel oxide, silver oxide orlead oxide as luminescence quenchers.

The following compositions may also be used

TABLE 2 Compound Wt (g) Compound Wt (g) Compound Wt (g) SiO₂ 55 SiO₂ 70SiO₂ 50 B₂O₃ 65 B₂O₃ 80 Be₂CO₃ 20 Na₂CO₃ 29 Na₂CO₃ 29 Sr₂CO₃ 20 K₂CO₃ 20K₂CO₃ 20 Na₂CO₃ 10 Li₂CO₃ 5 Li₂CO₃ 5 K₂CO₃ 10 Al₂O₃ 2 Al₂O₃ 2 Li₂CO₃ 5MgO 8 MgO 5 Al₂O₃ 2 MgO 5

TABLE 3 Compound Wt (g) Compound Wt (g) SiO₂ 35 SiO₂ 55 B₂O₃ 80 B₂O₃ 65Be₂CO₃ 40 Na₂CO₃ 29 Na₂CO₃ 29 K₂CO₃ 20 K₂CO₃ 20 Li₂CO₃ 5 Li₂CO₃ 5 Al₂O₃2 Al₂O₃ 2 MgO 8 MgO 8Another suitable composition is of the type

TABLE 4 Compound Wt % SiO₂ 51 B₂O₃ 13 Al₂O₃ 8 MgO 6 CaO 10 SrO 4 ZnO 4This is particularly suitable as a base for incorporating dopants forvisible wavelength absorption detection because all the base elementshave largely unfeatured absorption spectra.

Dopants have also been successfully incorporated into glass matriceswith the following ranges of chemical composition.

30–56 wt % SiO₂,

5–35 wt %, La₂O₃/Bi₂O₃/Sr₂O₃,

2–33 wt % Li₂O/K₂O/Na₂O,

0–6% Al₂O₃

wherein the La, Bi, Sr are examples of a suitable high Atomic numbercomponent.

Incorporation of all three alkaline earth compounds, plus BaO, givesmuch reduced melting temperatures.

Preferred elements for dopant fabrication for visible wavelengthabsorption system

TABLE 5 Barium Zinc Lanthanum Samarium Lead Praesodymium MagnesiumEuropium Strontium Boron-10 Titanium Neodymium Chromium Holmium IronThulium Caesium Cadmium Molybdemum Antimony Nickel Erbium TungstenLutecium Cobalt Tin Sodium Potassium Terbium

Improvements and modifications may be incorporated without deviatingfrom the scope of the invention.

1. A method of providing a document with a covert security feature inwhich the document is provided with at least one inorganic dopant, thedopant being of a material which can be identified by examination of itsvisible wavelength absorption spectrum, measured in either reflective ortransmissive mode, in response to broadband visible wavelength photonradiation, in which the dopant is fused with other elements andmicronised into a fine powder before being applied to or otherwiseincorporated into the document, thereby altering said visible wavelengthabsorption spectrum of the dopant, and in which the dopant exhibits noUV, visible or IR stimulated output, and in which the dopant is fusedinto a glass.
 2. A method of providing a document with a covert securityfeature in which the document is provided with at least one inorganicdopant, the dopant being of a material which can be identified byexamination of its visible wavelength absorption spectrum, measured ineither reflective or transmissive mode, in response to broad-bandvisible wavelength photon radiation, in which the dopant is fused withother elements and micronised into a fine powder before being applied toor otherwise incorporated into the document, thereby altering saidvisible wavelength absorption spectrum of the dopant, and in which thedopant exhibits no UV, visible or IR stimulated output, and in which thedopant is such that, when the document is illuminated with broad-bandvisible light the absorption features of said visible wavelengthabsorption spectrum are created at wavelengths to which the human eye isinsensitive.
 3. A method of providing a document with a covert securityfeature in which the document is provided with at least one inorganicdopant, the dopant being of material which can be identified byexamination of its visible wavelength absorption spectrum, measured inether reflective or transmissive mode, in response to broad-band visiblewavelength photon radiation, in which the dopant is fused with otherelements and micronised into a fine powder before being applied to orotherwise incorporated into the document, thereby altering said visiblewavelength absorption spectrum of the dopant, and in which the dopantexhibits no UV, visible or IR stimulated output; and in which the dopantis mixed with a quantity of an element with an atomic number greaterthan 36, or its salt or its oxide.
 4. A method of providing a documentwith a covert security feature in which the document is provided with atleast one inorganic dopant, the dopant being of a material having acomplex visible wavelength absorption spectrum including multipleidentifiable absorption features and which can be identified byexamination of said visible wavelength absorption spectrum, measured ineither reflective or transmissive mode, in response to broad-bandvisible wavelength photon radiation, in which the dopant is fused withother elements and micronised into a fine powder before being applied toor otherwise incorporated into the document, thereby altering saidvisible wavelength absorption spectrum of the dopant, and in which thedopant exhibits no UV, visible or IR stimulated output; and in which thedopant is fused in a glass.
 5. A method of making a dopant for use inproviding a document with a covert security feature, said dopant havinga complex visible wavelength absorption spectrum including multipleidentifiable absorption features and which can be identified byexamination of said visible wavelength absorption spectrum, measured ineither reflective or transmissive mode, in response to broadband visiblewavelength photon radiation, comprising fusing one or a combination ofthe element Barium, Zinc, Lanthanum, Samarium, Lead, Praseodymium,Magnesium, Europium, Strontium, Boron-10, Titanium, Neodymium, Chromium,Holmium, Iron, Thulium, Caesium, Cadmium, Molybdenum, Antimony, Nickel,Erbium, Tungsten, Lutetium, Cobalt, Tin, Sodium, Potassium, Terbium, inelemental form or as an oxide or salt, in a glass and subsequentlymicronising said glass into a fine powder, thereby altering said visiblewavelength absorption spectrum of the dopant, said dopant exhibiting noUV, visible or IR stimulated output.
 6. A method of providing a documentwith a covert security feature as claimed in claim 4, in which thedopant comprises one of, or a combination of the elements Barium, Zinc,Lanthanum, Samarium, Lead, Praseodymium, Magnesium, Europium, Strontium,Boron-10, Titanium, Neodymium, Chromium, Holmium, Iron, Thulium,Caesium, Cadmium, Molybdenum, Antimony, Nickel, Erbium, Tungsten,Lutetium, Cobalt, Tin, Sodium, Potassium, Terbium, in elemental form oras an oxide or salt.
 7. A method of providing a document with a covertsecurity feature as claimed in claim 3 in which the element or its saltor its oxide is Strontium, Lanthanum or Bismuth.
 8. A method ofproviding a document with a covert security feature as claimed in claim3, in which the dopant is mixed with ink and the resulting mixture isapplied to the document.
 9. A method of providing a document with acovert security feature as claimed in claim 4 in which the glass is madeof silicates and/or phosphates and/or borates.
 10. A method of providinga document with a covert security feature as claimed in claim 3, inwhich each particle of the micronised fine powder has a diameter of 1–4μm.
 11. A method of providing a document with a covert security featureas claimed in claim 3, in which the dopant is such that, when thedocument is illuminated with broad-band visible light to produce areflectance spectrum with frequency components generated by the dopantand by other reflecting substances contained in the document, saidspectrum, contains minimal frequency overlap between the components ofthe spectrum generated by the dopant and that part of the spectrumgenerated by other substances contained in the document.
 12. A method ofproviding a document with a covert security feature as claimed in claim3, in which the dopant is such that, when the document is illuminatedwith broad-band visible light the absorption features of said visiblewavelength absorption spectrum are created as wavelengths to which thehuman eye is insensitive.
 13. A method of providing a document with acovert security feature as claimed in claim 3, in which said visiblewavelength absorption spectrum of the dopant can be shifted to a higheror lower wavelength.
 14. A method of providing a document with a covertsecurity feature as claimed in claim 3, in which said visible wavelengthabsorption spectrum of the dopant can be shifted to a higher or lowerwavelength by alteration of the composition of the glass in which it isfused.
 15. A method of providing a document with a covert securityfeature as claimed in claim 3, in which said visible wavelengthabsorption spectrum of the dopant is alterable by alteration of thereaction temperature and/or pressure at which the glass is made.
 16. Adocument provided with a covert security feature by the method of claim3.
 17. A method of providing a document with a covert security featureas claimed in claim 4, in which the dopant is mixed with ink and theresulting mixture is applied to the document.
 18. A method of providinga document with a covert security feature as claimed in claim 4, inwhich each particle of the micronised fine powder has a diameter of 1–4μm.
 19. A method of providing a document with a covert security featureas claimed in claim 4, in which the dopant is such that, when thedocument is illuminated with broad-band visible light to produce areflectance spectrum with frequency components generated by the dopantand by other reflecting substances contained in the document, saidspectrum contains minimal frequency overlap between the components ofthe spectrum generated by the dopant and that part of the spectrumgenerated by other substances contained in the document.
 20. A method ofproviding a document with a covert security feature as claimed in claim4, in which the dopant is such that, when the document is illuminatedwith broad-band visible light the absorption features of said visiblewavelength absorption spectrum are created at wavelengths to which thehuman eye is insensitive.
 21. A method of providing a document with acovert security feature as claimed in claim 4, in which said visiblewavelength absorption spectrum of the dopant can be shifted to a higheror lower wavelength.
 22. A document provided with a covert securityfeature by the method of claim 1.